Update implementation status docs

- Mark the user signature upload API as implemented in the backendization plan.

- Refresh the progress next-step list to focus on remaining E2E seed migration, fallback cleanup, audit visibility, export API, rate limits, data migration, and production operations.

docs/backendization-plan.md

@@ -204,7 +204,7 @@
 - `GET /api/users/:id`
 - `PATCH /api/users/:id`
 - `DELETE /api/users/:id`
-- `POST /api/users/:id/signature`（待实现）
+- `POST /api/users/:id/signature`
 - `GET /api/departments`
 - `POST /api/departments`
 - `PATCH /api/departments/:id`